Venetoclax overcomes resistance to all‐trans retinoic acid in a variant acute promyelocytic leukemia with TNRC18::RARA fusion

Acute promyelocytic leukemia (APL) is generally driven by PML::RARA, but approximately 2% of variant APL patients do not contain this fusion gene and pose challenges in diagnosis and treatment. Here, we reported an aggressive APL patient with variant TNRC18::RARA fusion gene, who was resistant to standard differentiation induction therapy consisting of all‐trans retinoic acid (ATRA) and arsenic trioxide but achieved complete remission with venetoclax plus ATRA. Mechanistically, venetoclax possesses synergistic effects in ATRA‐induced TNRC18::RARA‐positive cell differentiation.

Variant acute promyelocytic leukemia (APL) shares morphological, immunophenotypic, and clinical features with typical APL, but lacks the PML::RARA fusion gene.Most variant APLs are resistant to therapy with all-trans retinoic acid (ATRA) and arsenic trioxide (ATO). 1 Here, we discuss a woman with a novel trinucleotide repeat containing 18 (TNRC18)::RARA fusion unresponsive to ATRA.
A 57-year-old woman with diabetes was admitted because of gum bleeding and fatigue for 2 months.Ten days before admission, she was diagnosed with APL and received ATRA and ATO with no response in a local hospital.Reverse transcription polymerase chain reaction (RT-PCR) analysis failed to detect the PML::RARA fusion transcript in the bone marrow sample and she was referred to our hospital.Bone marrow aspirate showed 30% abnormal hypergranular promyelocytes with Auer rods (Figure 1A).The blasts were strongly positive for myeloperoxidase.Flow cytometry studies indicated strong expression of CD117, CD123, CD13, CD33, CD56, and cMPO, weak expression of CD34 and HLA-DR, and no expression of CD11b, CD14, CD15, CD36, CD64, cCD3, CD5, CD7, CD19, or cCD79a (Figure S1A).The karyotype was 46, XX, t(7:17) (p22:q21), del(11) (p11.Treatment with ATRA (20 mg twice daily) and ATO (10 mg per day) was continued and daunorubicin (20 mg per day) was added.
After 1 month of treatment, there were still 45.5% of promyelocytes remained in the bone marrow and the patient developed hemoptysis despite the strong support of blood products (Figure 1C).ATO and daunorubicin were stopped and venetoclax was started.Remarkably, fibrinogen levels gradually increased independent of blood transfusions, and complete remission (CR) was finally achieved 22 days after the incorporation of venetoclax with reference to morphology, flow cytometry, and cytogenetics (Figure 1D).The patient continued with oral administration of ATRA and venetoclax after discharge and received consolidation therapy with ATRA, ATO, daunorubicin, and venetoclax followed by an allogeneic hematopoietic cell transplant (allo-HSCT), and she remains in remission as of November 2023 (Figure 1C).
To search for the potential fusion gene, RNA sequencing of bone marrow samples was done with a HiSeq (Illumina.Inc.) and the TNRC18::RARA fusion gene was detected.We confirmed the finding by RT-PCR using complementary DNA (cDNA) of bone marrow cells and these primers: forward (at TNRC18 exon 5), 5′-CAGGGT GAGGCAGAAGTG-3′ and reverse (at RARA exon 3), 5′-GACAAA GCAAGGCTTGTAGATG-3′.As expected, a band of approximately 231 bp was visualized by electrophoresis in the bone marrow sample but not in the MOLM-13 AML cell line.A reciprocal RARA::TNRC18 transcript was not detected (Figure S1C).Using Sanger sequencing we found an in-frame fusion between exon 5 of TNRC18 and exon 3 of RARA.
For functional analyses, full-length TNRC18::RARA cDNA was amplified from the patient's bone marrow sample and cloned into a pMIG expression vector with FLAG-tag.U937, a human monocytic cell line, stably transfected with fusion proteins was constructed and used to detect the effect of drug treatment (Figure S1D).TNRC18::RARA-expressing U937 cells were less sensitive to ATRA than vector-transfected U937 cells (Figure 1E).Overexpression of TNRC18::RARA impeded ATRA-induced U937 differentiation as assessed by the expression of myeloid lineage markers CD11b and CD33 (Figure 1F).However, coadministration of venetoclax and ATRA could enhance ATRA-induced cell differentiation evidenced by immune phenotype and morphology (Figure 1G,H).Consistently, the expressions of differentiation-related genes, such as colonystimulating factor 3 receptor (CSF3R), RUNX family transcription factor 1 (RUNX1), and CCAAT enhancer binding protein beta (CEBPB) were upregulated to a much greater extent in the combinational group (Figure 1I).Together, these data indicate venetoclax is active in an ATRA-induced TNRC18::RARA-positive cell differentiation.
To our knowledge, this is the second report of a very aggressive case of APL with TNRC18::RARA.Breakpoints are the same as reported by Wang et al. 2 TNRC18::RARA binds to retinoic acid response elements (RARE) as a homo-and a heterodimer with retinoic X receptor α (RXRA) and has a dominant negative action on transcription, thereby impeding cellular differentiation. 2How to treat APL patients with TNRC18::RARA remains challenging because of severe coagulation disorders and poor response to ATRA and ATO.In particular, these two cases are both classified as the CD56-positive APL subtype, which has been demonstrated to be associated with poor prognosis. 3In the previous report, a quick switch to conventional idarubicin, cytosine arabinoside, and ATRA benefited the patient.We claimed that low-dose venetoclax plus ATRA is another feasible option and allo-HSCT may improve long-term survival.
Venetoclax, an oral, potent, and selective B-cell lymphoma 2 (BCL-2) inhibitor, has been widely used as the front-line treatment for elderly or unfit AML patients.Venetoclax-based treatment was also shown to be an effective salvage therapy for ATRA/ATO-resistant APL patients. 4,5][8][9][10] This is the first study to report on the efficacy and possible mechanisms of venetoclax plus ATRA regimen as a differentiation induction therapy in patients with variant APL carrying TNRC18::RARA.Venetoclax in combination with ATRA possesses advantages in terms of time to CR compared to venetoclax monotherapy and safety profiles compared to venetoclaxbased chemotherapy, especially suitable for older patients who are ineligible for intensive chemotherapy.
In summary, our case provided evidence that venetoclax combined with ATRA followed by allo-HSCT is effective for treating APL with TNRC18::RARA.The individual described herein is now over a year out from the completion of therapy without any evidence of disease recurrence.Since there is no unified consensus on the management of variant APL, it would be interesting to determine whether this therapeutic regimen would be effective for other subtypes of variant APLs.Abbreviations: ATRA, all-trans retinoic acid; ATO, arsenic trioxide; allo-HSCT, allogeneic hematopoietic stem cell transplantation; CR, complete remission; NA, not available.
Variant APL treated with venetoclax-based therapy in literature.
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